A heart murmur in cats is an abnormal sound your vet hears through a stethoscope during a routine exam. It’s caused by turbulent blood flow inside the heart or nearby blood vessels, which creates audible vibrations that wouldn’t be present if blood were flowing smoothly. A murmur isn’t a disease itself. It’s a physical finding that can point to heart disease, a non-cardiac condition like an overactive thyroid, or sometimes nothing concerning at all.
Why Murmurs Happen
In a healthy heart, blood flows in smooth, orderly streams (called laminar flow) and makes no extra sound. A murmur occurs when something disrupts that flow and creates turbulence, the same way water rushing past a rock in a stream makes noise that calm water doesn’t. That disruption can come from a structural problem inside the heart, like a thickened wall or a leaky valve, or from changes in how fast blood is moving through otherwise normal structures.
In cats specifically, the most common cause of a pathologic murmur is hypertrophic cardiomyopathy (HCM), a condition where the heart muscle thickens abnormally. This thickening narrows the space blood has to pass through, forcing it into turbulent patterns. But many cats with murmurs have no structural heart disease at all. Their murmurs come from a temporary increase in blood flow speed through the right side of the heart, a mechanism called dynamic right ventricular outflow tract obstruction. This is the main way “innocent” murmurs are generated in cats.
Innocent Murmurs vs. Pathologic Murmurs
Not every murmur means your cat has heart disease. Cats are unusual among pets because systolic heart murmurs (the most common type, occurring when the heart contracts) are frequently heard in cats with completely normal hearts. Kittens under three months old sometimes have innocent murmurs that disappear as they grow, likely caused by the proportionally high volume of blood moving through still-small vessels.
In adult cats, non-disease murmurs tend to be quiet, graded between 1 and 3 on a 6-point scale. The challenge is that murmurs in this softer range can’t reliably be classified as innocent or pathologic by sound alone. A soft murmur could be completely harmless or the first sign of early heart disease. Louder murmurs, graded 4 through 6, almost always indicate structural heart disease.
Several non-cardiac conditions can also produce murmurs by changing how blood flows through the heart. Anemia makes the heart pump harder to compensate for fewer oxygen-carrying red blood cells. Hyperthyroidism, common in older cats, stimulates the heart to beat faster and contract more forcefully, and over time can cause thickening of the left ventricle. In these cases, treating the underlying condition often resolves or reduces the murmur.
How Murmurs Are Graded
Vets rate murmur intensity on a scale from 1 to 6. A grade 1 murmur is barely audible, detectable only in a quiet room with careful listening. A grade 6 is so loud it can be heard with the stethoscope barely touching the chest. At grades 5 and 6, the turbulence is intense enough that the vet can actually feel a vibration (called a thrill) by placing a hand on the cat’s chest wall.
Louder doesn’t always mean worse in a perfectly linear way. Grades 1 and 2 usually indicate either no heart disease or mild disease. Grades 3 and 4 fall into an unpredictable middle zone where the loudness alone doesn’t reliably predict severity. But grades 4 through 6 generally point to structural heart disease that needs further investigation.
Signs to Watch For
Many cats with heart murmurs show no symptoms, especially early on. Cats are notoriously good at hiding illness, and heart disease can progress silently for months or years before outward signs appear. When symptoms do develop, they reflect the heart’s declining ability to pump blood effectively or complications like fluid buildup and blood clots.
Signs of advancing heart disease include:
- Faster breathing at rest (more than 30 to 40 breaths per minute while sleeping)
- Visible effort to breathe, with exaggerated chest or belly movement
- Panting or open-mouth breathing after mild activity
- Chronic weight loss or muscle wasting
- Decreased appetite or increased hiding
- Lethargy or weakness
- Gums that appear blue, gray, or white instead of pink
The most alarming complication is a blood clot (thromboembolism) that breaks loose from the heart and lodges in a blood vessel, most commonly cutting off blood flow to the hind legs. This causes sudden paralysis of the back legs, often with painful vocalization. It’s a medical emergency. In kittens with severe congenital heart defects, stunted growth can be an early indicator.
How Vets Diagnose the Cause
Hearing a murmur tells your vet something is worth investigating, but it doesn’t reveal the cause. The next step depends on the murmur’s grade and your cat’s overall health.
Chest X-rays can show whether the heart is enlarged and whether fluid has accumulated in or around the lungs. They’re useful for getting a general picture, but they’re far less precise than ultrasound at measuring individual heart chambers or identifying specific structural problems.
An echocardiogram (heart ultrasound) is the gold standard for diagnosing what’s behind a murmur. It lets the vet see the heart in real time: measuring wall thickness and chamber sizes, watching how valves open and close, and calculating blood flow speeds. A specialized version called Doppler echocardiography tracks the movement of red blood cells to pinpoint exactly where turbulent or high-speed flow is occurring. This is what confirms or rules out conditions like HCM, valve disease, or congenital defects.
Your vet will likely also run blood work. A complete blood count checks for anemia, and a thyroid panel screens for hyperthyroidism, both of which can cause murmurs without any heart disease being present.
Breeds With Higher Risk
Hypertrophic cardiomyopathy has a genetic component in several breeds. Maine Coons and Ragdolls are the best-studied, with identified gene mutations that increase HCM risk. Persians, Sphynx, and British Shorthairs also appear to have elevated rates. Siamese and Burmese cats carry higher risk for certain congenital heart defects. Mixed-breed cats can develop HCM too, but purebred cats in high-risk breeds benefit from earlier and more regular screening.
What Happens After Diagnosis
If the murmur turns out to be innocent, or caused by a treatable condition like hyperthyroidism, no heart-specific treatment is needed. Your vet may recommend periodic rechecks to make sure nothing has changed.
If heart disease is confirmed, treatment depends on how far it has progressed. Cats with early, subclinical disease (a structural abnormality but no symptoms) may not need medication right away. Monitoring becomes the priority: periodic echocardiograms and tracking resting respiratory rate at home. Counting your cat’s breaths per minute while they sleep is one of the simplest and most useful things you can do. A consistent increase above their baseline often signals fluid buildup before other symptoms appear.
When heart failure develops, meaning the heart can no longer keep up with the body’s demands, medication becomes necessary. Treatment typically involves drugs that remove excess fluid from the lungs and body, medications that relax blood vessels to reduce the heart’s workload, and sometimes drugs to control abnormal heart rhythms. These are usually oral medications given once or twice daily at home, and finding the right combination and dose for your cat can take some adjustment over time.
Life Expectancy With Heart Disease
Prognosis varies enormously depending on the type and stage of disease. A study of 260 cats with HCM published in the Journal of the American Veterinary Medical Association found that cats diagnosed before symptoms appeared had a median survival time of about 1,129 days (roughly three years) from diagnosis. Cats who developed heart failure had a median survival of 563 days (about a year and a half). Cats whose first sign was a blood clot had the shortest median survival at 184 days, though individual outcomes ranged widely, with some cats living years beyond their diagnosis.
These numbers represent cats with confirmed HCM, the most common form of heart disease in cats. Many cats with low-grade murmurs and no underlying structural disease live completely normal lifespans without ever developing problems. The murmur grade at the time of discovery doesn’t determine outcome nearly as much as what’s causing it and how early it’s caught.